1. Field of the Invention
Embodiments of the invention relates to a display device, and more particularly, to a liquid crystal display device and a method of driving the same. Although embodiments of the invention are suitable for a wide scope of applications, they are particularly suitable for restricting viewing angle.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) includes a first substrate, a second substrate, and a layer of liquid crystal molecules. The first and second substrates face each other and are spaced apart from each other. The layer of liquid crystal molecules is interposed therebetween. The LCD device uses optical anisotropy and polarization properties of liquid crystal molecules to display images.
The liquid crystal molecules have orientation characteristics resulting from their thin long shape. Thus, an arrangement direction of the liquid crystal molecules can be controlled by applying an electrical field across the liquid crystal molecules. A thin film transistor (TFT) can be used as a switching element to control application of the electric field. An LCD device using a TFT is referred to as an active matrix LCD (AM-LCD) device, which has excellent characteristics of high resolution and being able to smoothly display moving images.
A related art LCD device includes a first substrate, a second substrate and a layer of liquid crystal molecules. The first substrate includes a pixel electrode, and the second substrate includes a color filter layer and a common electrode. The first and second substrates face each other, and the layer of liquid crystal molecules is interposed therebetween. The related art LCD device displays images using a vertical electric field between the pixel and common electrodes. The LCD device using the vertical electric field has a high transmittance and a high aperture ratio.
The LCD device using the vertical electric field has the problems of narrow viewing angle and low contrast ratio. To resolve the above-mentioned problems, new display devices, such as an in-plane switching (IPS) mode LCD device, a vertical alignment (VA) mode LCD device and a fringe field switching mode LCD device can be used instead of a vertical electric field LCD. These LCD devices are explained in reference to FIGS. 1 to 3.
FIG. 1 is a cross-sectional view of an IPS mode LCD device according to the related art. As shown in FIG. 1, the IPS mode LCD device includes first and second substrates 10 and 20 and a layer of liquid crystal molecules 30 interposed therebetween. The first substrate 10 includes pixel and common electrodes 12 and 14 such that a horizontal electric field 40 is induced between the pixel and common electrodes 12 and 14 using voltages applied to the pixel and common electrodes 12 and 14. The IPS mode LCD device uses the horizontal electric field 40 to display images. Since there is little variance in the refractive index of the liquid crystal molecules in the light transmission path while switching the IPS mode LCD device, a wide viewing angle is maintained.
FIG. 2A is a cross-sectional view of a VA mode LCD device according to the related art. As shown in FIG. 2A, the VA mode LCD device includes first and second substrates 10 and 20 and a layer of liquid crystal molecules 30 interposed therebetween. The pixel and common electrodes 12 and 24 are formed on the first and second substrates 10 and 20, respectively. A pixel electrode slit 12a and a common electrode slit 14a are formed on the pixel and common electrodes 12 and 24, respectively. An electric field 50 induced between the pixel and common electrodes 12 and 24 is distorted by the pixel electrode slit 12a and the common electrode slit 14a such that first and second oblique electric fields 50a and 50b are formed. As a result, the VA mode LCD device has two domains that result in a wide viewing angle.
FIG. 3 is a cross-sectional view of an FES mode LCD device according to the related art. As shown in FIG. 3, the FES mode LCD device includes first and second substrates 10 and 20 and a layer of liquid crystal molecules 30 interposed therebetween. The common electrode 14, a gate insulating layer 16 and a plurality of pixel electrodes 12 having a bar shape are sequentially formed on the first substrate 10. The common electrode 14 is formed over the entire surface of the first substrate 10. The plurality of pixel electrodes 12 are separated from each other. The gate insulating layer 16 between the common electrode 14 and the pixel electrodes 12 prevents a short between the common and pixel electrodes 14 and 12. Since the pixel and common electrodes 12 and 14 are formed on the first substrate and of different layers, a fringe field 60 is formed between the pixel and common electrodes 12 and 14. The fringe field 60 has a reverse U-shape. Since the reverse U-shape has a flat top portion, the FES mode LCD device functions like the VA mode LCD device so as to have a wide viewing angle.
The above-mentioned LCD devices having a wide viewing angle that also resolves the problems of color shift and gray inversion at side viewing angles. A wide viewing angle is useful in that many users at different viewing angles can observe the same image. However, there are situations in which only one viewer should see the image. For example, a person using an automatic teller machine should be the only person able to view the image. In this case, the viewing angle should restricted. A method of restricting the viewing angle with a filter has been used in the past. However, the filter method increases production time and production costs of the LCD device as well as dims the brightness of the display.